Extended Objects

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Extended Objects: Europa Observations

Europa, satellite of Jupiter was observed on November 18, 2013 from 08:42:16 (airmass=1.63) to
08:48:16 (airmass=1.64) under average seeing conditions (DIMM seeing~0.6-0.8) in K1 band, and direct mode (no coronagraph, no apodizer). Eleven data cubes with a true integration time of 8.73 s
were processed by M. Perrin using the recipe listed below.

Recipy used to process Europa Data

Load Wavelength Calibration

Subtract Dark Background

Update Spot Shifts for Flexure

Destripe science image

Remove_microphonics

Interpolate bad pixels in 2D frame

Assemble Spectral Datacube

Interpolate Wavelength Axis

Divide by Lenslet Flat Field

Interpolate bad pixels in cube

Rotate North Up

Accumulate Images

Combine 3D datacubes

Ephemeris calculations provided by JPL & IMCCE indicate that the satellite was bright (V~5.6) with an apparent diameter of 0.9508 arcsec and observed with a phase angle of
8.9 deg, leading to an illumination fraction of 99.4%. Europa disk is
therefore covering ~10% of the detector in an area almost centered on the detector.
The last 6 images were taken with CCR at low power and visually seem to be of better quality. It is not possible to quantify the resolution on these observations and the
gain with CCR at low power since Europa does not display unresolved features necessary to perform a direct measurement.
Europa was not dithered on the set of
observations. We estimated the residual jittering to be 0.12 pixel in
average by fitting the disk with an ellipsoid and measuring its
center for each frame. Figure below shows the astrometric position of
Europa (labeled from E0 to E10), plus the residual jittering measured
on seven frames of HD1160, a A0 V=7.1 star observed in K1 direct
imaging mode on the same night between 01:20 and 01:35 UT (airmass
~1.2, DIMM seeing~0.8 arcsec), label P0 to P5. Because the residual
jittering on the “PSF” stars is slightly higher (0.31 pixel in
average), we conclude that no residual jittering due to the extended
angular size of Europa is apparent. It is likely that the angular
resolution on these images of Europa is in fact very close to the
angular resolution that we measured on the “PSF”, so 65 +/- 1
mas.

After data processing, the frames in the datacube still display some imperfections visible as 15-20 pixels of low and high intensity on Europa’s disk. Most of these bad pixels are identical on the 11 observations and are probably due to bad pixels on the detector.
Additionally, some of the frames in the cube contains an oblique
pattern a misalignment between the spectra position in the calibration and the actual position in the detector image. This misalignment results in a poor flux extraction when the cube is
assembled.

Pixel Scale from Europa Observations.

We estimated the pixel scale on final
assembled cube by fitting the observations of Europa by a sphere
illuminated under the same geometry than Europa (Sub-Solar point and
Sub-Earth point from the ephemeris) and degraded after convolving it
by a gaussian PSF with a FWHM of 65 mas. We generated a grid of
solutions varying the Minnaert coefficient which defines the
center-to-limb profile from 1.1 to 1.5 (step of 0.02) and the pixel
scale from 13 to 15 mas (step of 0.04). The best fit is obtained with
a pixel scale estimated to 14.30 ± 0.31.

Spectroscopic variations

A 3-color picture (1.95, 1.99,2.18 um)
included below shows variegation on Europa’s disk. A comparison
with visible observations based on Galileo-Voyager spacecraft
suggests an anti-correlation between the dark albedo features in
visible of Europa which are bright at ~2 um.

To confirm that these spectroscopic
differences are real, hence not instrument artifacts, we extracted
the spectra of 3 areas which show significant variations on the
complete data set of Europa. The eastern limb is bright at ~1.8um,
the bright center area is bright at ~2um and an isolated patch
north-west of Europa that could be a young crater. The flux of the
extracted spectra is normalized after dividing it by the flux of
HD1160 an A0 star without including an airmass correction.

The extracted spectra of these 3 different area on Europa are similar among all the spectro-images
within 3%, confirming the stability of the instrument.
The spectra of these three regions display identical absorption bands
at 1.95, 2.00, 2.05 um which could be of atmospheric origin. It is
however clear that spectroscopic variabilities (e.g at lambda>2.1
um) are detectable on Europa disk.

Neptune observations

Neptune (JPL ephemeris V=7.9, angular diameter = 2.26 arcsec) was observed on Dec 11 2013 from 00:17 to 00:31. Seven frames with an exposure time of 59.65s were recorded in
H direct with an airmass of 1.3 and a DIMM seeing of 0.66 arcsec. This data is still being processed.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five participant countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.